Information is transmitted in mobile telephone systems using a multiple access technique. Some systems use frequency division multiple access (FDMA) and time division multiple access (TDMA), users of the network being distinguished from each other by the respective frequency used and by the information to be transmitted being delivered in time slots assigned to each user. In systems based on code division multiple access (CDMA), users communicate with each other using the same radio frequency band. To be able to distinguish users from one another, each is assigned a respective spreading code for the whole duration of a connection, the code being used to spread the spectrum of the signal to be transmitted in base band. To reconstitute the information transmitted, the receivers must use the same code to effect the operation that is the converse of the spreading operation. Compared to other multiple access methods, this technique has the advantage of being more flexible in terms of access and bit rate, which can be varied by altering the spreading factor.
In radio transmission, the form of the medium between the sender and the receiver of a radio signal interferes with transmission and leads to propagation along multiple paths caused by reflections at different points along the radio channel, especially in an urban environment. As a result, components of the same signal reach the receiver with different powers and different time delays.
CDMA receiver systems use a rake receiver to reconstitute the transmitted signal from the components received over different propagation paths. These receivers are based on reconstituting a delay profile or radio channel equivalent model. To this end, a sequence of pilot symbols known to the receivers is transmitted together with the information, and on the basis of this prior knowledge the receivers perform an estimation (an impulse response representing all the paths of the radio channel) of the radio channel over which the received signal was transmitted. A matched filter is shifted over the received signal, for example by half a spreading code unit, while the received power is measured. This technique is used to construct an impulse response graph giving information on the power and the time delays caused by multipath propagation of the components of the signal received over a given radio channel.
Although the CDMA technique would seem to be very suitable for real time low-bit-rate services, it appears to be unsuitable for high-bit-rate packet services because the performance of the rake receiver is based on cross-correlation and autocorrelation properties of the spreading sequences, which improves as the length of the spreading sequence, and thus of the spreading factor, is increased. Now, the higher the bit rate, the lower the spreading factor. The spreading sequence becomes shorter and the cross-correlation and autocorrelation properties of the spreading sequences are therefore degraded, leading to interference between symbols of the same transmitted signal. As a result of this, the performance of the rake receiver is seriously degraded for spreading factors less than 8, especially if the type of modulation used has a large number of states.
A study of the degraded performance of rake receivers caused by intersymbol interference has shown that it is necessary to use an equalization technique if the spreading factor is less than 16 (see [1] “On the rake receiver performance” H. Boujemaa, M. Siala, VTC 2000 Fall, Boston, USA).
Thus rake receivers on their own are found to be a very unsuitable response to the requirements of high-bit-rate mobile telephony.
At present it is virtually impossible to use optimum detection and encoding techniques, as this leads to very high calculation complexity, especially if the transmission channels have an impulse response that is too long, as is the case in an urban environment.
Various suboptimal detection and decoding methods are used in the time division multiple access (TDMA) technique.
For example, the technique using a linear minimum mean square error equalizer (LMMSE) reduces not only intersymbol interference but also interuser interference. For more, details, see the documents:                [2] “Linear receivers for the DS-CDMA downlink exploiting orthogonality of spreading sequences” by I. Ghauri and D. T. M. Slock, in Proc. 32nd Asilomar Conf. on Signals, Systems and Comp., Asilomar, Calif., Nov. 1-4 1998, and        [3] “Interference Suppression in CDMA Downlink through Adaptive Channel Equalization”, by M. Heikkilä, P. Komulainen, and J. Lilleberg, in VTC 99 Fall, Tokyo, Japan.        
Note that interuser interference on downlinks is caused by multipath propagation in the channels, given that the spreading sequences of different users are mutually orthogonal. That solution proves to be very effective at reducing interuser interference, compared to rake receivers. However, because of the linear characteristics of the LMMSE equalizer, that solution does not significantly reduce intersymbol interference.
It has also been proposed to provide a maximum likelihood sequence estimation equalizer (MLSE) at the output of a rake receiver. For more details of that technique see for example the document [4] “Joint multipath combining and MLSE equalization (rake-MLSE Receiver) for WCDMA systems” by S. Tantikovit, and A. U. H. Sheikh, in VTC 2000 Spring, Tokyo, Japan.
That solution is optimized from the sequence-detection point of view, and close to the optimum solution in terms of detecting errors in the symbols transmitted. However, the complexity of that solution increases exponentially with the spreading of the time delays in the same channel and with the size of the modulation constellation employed. Thus it cannot be applied to all UMTS services. Furthermore, that solution does not take into account the degraded performance caused by incorrect channel estimation and channel coding. Furthermore, it does not offer a flexible or weighted output algorithm.
The prior art delayed decision feedback sequence estimation (DDFSE) technique reduces the complexity of the states of the trellis by using the “per survivor” processing technique. For more details of that technique, see for example the document [5] “Delayed Decision-Feedback Sequence Estimation” by A. Duel-Hallen and C. Heegard, in IEEE Transactions on Communications, Vol. 37, pp. 428-436, May 1989.
In TDMA systems, that technique has the drawback of being sensitive to error propagation, which necessitates prefiltering. That technique appears to be inapplicable to CDMA systems since the channel equivalent model at the rake receiver output varies on each symbol transmitted, as it depends on the spreading code, which changes on each symbol.
An iterative detection and decoding method designed for the TDMA technique and known as “turbo-detection” is described in the paper [6] “Iterative Correction of Intersequential Interference: Turbo-equalization” by C. Douillard, M. Jezequel, C. Berrou, A. Picart, P. Didier and A. Glavieux, published in European Transactions on Telecommunications, Vol. 6, p. 507 to 511, September 1995. In that detection and decoding technique, an MLSE equalizer with weighted inputs and outputs (SISO MLSE) is used, and the decoding process is of the Viterbi type, also with weighted inputs and outputs (SOVA). That process is described in a paper [7] entitled “A low Complexity Soft Output Viterbi Decoder Architecture”, ICC '93 p. 733 to 740, Geneva, Switzerland, May 1993.
The above detection and decoding technique has been further developed, yielding optimized maximum a posteriori probability (MAP) detectors. For more details of those detectors, see the following papers:                [8] “Optimal Decoding of Linear Codes for Minimizing Symbol Error Rate” published by L. R. Bahl, J. Cocke, F. Jelinek and J. Raviv in IEEE Transactions on Information Theory, Vol. IT-20, p. 284-287, March 1994; and        [9] “Iterative Equalization and Decoding in Mobile Communications Systems”, published by G. Baush, H. Khorram and J. Hagenauer in Proc. EPMCC '97, p. 307-312, Bonn, Germany, September 1997.        
However, the above solution has not been transposed to CDMA systems, and further introduces undue complexity of the receiver, of the order of ML, where M is the number of points of the modulation constellation and L is the number of echoes in the propagation channel taken into account. Moreover, it does not address the channel estimation problem.
Finally, the paper [10] “Turbo-Equalization over Frequency Selective Channel”                International Symposium on Turbo-Codes, Brest, France, September 1997, proposes an iterative symbol detection and channel decoding technique, known as “turbo-equalization” and significantly different from the turbo-detection technique mentioned above, and which presupposes a noisy estimation of the transmission channel. However, compared to the turbo-detection technique, the turbo-equalization technique degrades performance in a way that is strongly dependant on the equalization technique employed for the first iteration. On that subject, see the paper [11] “Joint Equalization and Decoding: Why Choose the Iterative Solution?” by A. Roumy, I. Figalkow and D. Pirez, in IEEE VTC '1999 Fall, Amsterdam, Netherlands, September 1999.        
That technique cannot be transposed to CDMA systems since it is based on filtering techniques that cannot be applied if the channel varies independently from one transmitted symbol to another.